Some of the stumbling blocks you'll run into that aren't covered under WinForms development are:
- The ASP.NET Page Life Cycle
- IIS Configuration
- Security
- Performance
- Search Engine Optimization
- The HTTP protocol
- HTML/XML/CSS/Javascript
- Databases (most web sites anyways)
- Web Services
- Browser differences (this one really blows)
See What should a developer know before building a public web site? for a good start on what you'll be getting into.
